1. Field of the Invention
The present invention relates to a developing apparatus for use in an apparatus utilizing an electrophotography method or electrostatic recording method such as a copying machine, a printer, a facsimile machine or the like.
2. Related Background Art
In the image forming apparatus using an electrophotography method such as a copying machine or the like, an electrostatic latent image formed on an image bearing member such as a photosensitive drum or the like is visualized with a developer applied thereto. The developer includes a magnetic single component developer including a magnetic toner, a non-magnetic single component developer including a non-magnetic toner or a two component developer including a non-magnetic toner and a magnetic carrier, one of which will be used in compliance with circumstances.
Among conventional apparatuses that are used in connection with the aforementioned developers, an apparatus that uses a two component developer including a toner and a carrier is shown in FIG. 7. Many developing apparatuses using two component developer like the developing apparatus 102 shown in FIG. 7 have a single sleeve developing apparatus structure including a single developing sleeve 8 serving as a developer bearing member and transporting screws 5 and 6 serving as transporting means for transporting the two component developer while agitating it.
In such a developing apparatus, a blank area in an image sometimes occurs. The blank area (edge effect) is one of the adverse effects on images caused by edge enhancement.
In the following, the mechanism of generation of the blank area will be described with reference to FIG. 8. The description will be directed to the case in which a reversal development method is used. In general, a blank area in an image appears at a boundary of a highlight image portion and a solid image portion in a halftone image. Specifically, the blank area appears at a position between the trailing edge of the highlight image portion and the leading edge of the solid image portion. FIG. 8 shows the state of equipotential surfaces and electric flux lines in the case that a highlight portion is present on a photosensitive drum and a solid portion is present on the trailing side of the highlight portion, wherein the boundary portion between the highlight portion and the solid portion is opposed to a developing sleeve. It will be understood from FIG. 8 that the electric flux lines at the boundary portion are greatly attracted toward the solid portion.
Therefore, in the structure used in connection with the conventional developing method in which the developing sleeve is rotated in the forward direction, toner contained in the supplied developer is not supplied to the trailing edge of the highlight portion but supplied to the solid portion for development along the electric flux lines. Thus, it is considered that a blank area sometimes occurs at the trailing edge of the highlight portion.
In order to avoid the aforementioned blank area in an image, there has been proposed a twin sleeve developing method in which two developing sleeves are provided in the upstream side and the downstream side with respect to the rotation direction of the photosensitive drum respectively and the same single electrostatic latent image on the photosensitive drum is developed by a first developing process using the developing sleeve on the upstream side and a second developing process using the developing sleeve on the downstream side. In this twin sleeve developing method, the potential difference between the highlight portion and the solid portion is decreased in the first developing process and the trailing edge area of the highlight portion is sufficiently developed in the second developing process. Therefore, the blank area is rarely generated in the twin sleeve developing method.
However, in the aforementioned conventional developing apparatus, among a plurality of magnetic poles of magnets provided in the interior of the developing sleeve on the downstream side, the magnetic pole closest to the photosensitive drum (the so-called developing magnetic pole) provides a magnetic flux density substantially equal to the magnetic flux density of the developing magnetic pole of the developing sleeve on the upstream side, so that the bristles or the so-called magnetic brush made by the developer has a length equal to the length of the magnetic brush on the upstream side. Consequently, the toner image that has been developed in the first developing process by the developing sleeve on the upstream side is rubbed by the magnetic brush again in the second developing process by the developing sleeve on the downstream side with a substantially equal pressure. In that case, a scavenging phenomenon can occur, and image quality is sometimes greatly deteriorated.
In order to solve the above-described problem, in recent years, utilization of a carrier with a high magnetization and a low resistance as a magnetic carrier in the developer has been contemplated. First, with a reduction of the magnetization amount of the carrier, the length of the magnetic brush can be decreased, so that a force with which the magnetic brush rubs the toner image developed on the photosensitive drum is weakened. Thus, it is possible to enhance image quality. In addition, with an increase in the resistance, leakage of the charge is eliminated even in the case that the charge of the latent image on the photosensitive drum is rubbed by the magnetic carrier. Therefore, enhancement of image quality is attained for the reason that the digital electrostatic latent image is not disturbed.
However, in the case that the magnetization amount is reduced and the developing sleeve and the photosensitive drum are rotated in the respective forward directions, the length of the magnetic brush will become short and the nip width (in the circumferential direction) throughout which the developer is in contact with the photosensitive drum will become narrow. As a result, edge enhancement such as so-called sweeping-together, which is the phenomenon that the density of the trailing edge of a solid image portion is increased, becomes visible. In addition, with an increase in the volume resistance of the magnetic carrier, there arises a problem that the opposed electrode effect is weakened and the blank area level (or the degree of appearance of blank areas) is made worse.
In addition, since the magnetic carrier, the toner on which has been consumed, has an electric charge with the polarity reverse to the charge of the toner (so-called counter charge), in the case that the magnetic carrier, the toner on which has been consumed, stays in the developing nip for a long time, it has an effect of unsticking the toner developed on the photosensitive drum (that is, a scavenging effect caused by the counter charge). This may possibly bring about the situation that the blank area level is made worse. Especially, in the case that the aforementioned carrier with a high resistance is used, the above-mentioned situation becomes notable since a charge decay time is required.